There is another wqy to add sheers behind your drapes and much less expensive than the 'Add a Rod' idea as well as less cumbersome than the bungee idea, depending on the type of rod you use for the drapes. IF you use a standard type curtain rod, there is a solid 'u' shaped hook you can insert into the bottom fold of the of the rod while inserting a regular cafe type rod into it to hold the sheers. They can be found at big box stores and often at drapery stores. lollll But I can't find it on the Internet drapery ads. Mucho inexpensive, mucho easy, and your sheers will definitely be behind your drapes as far as you want.

If you want to do sheers under a curtain and you don't need them to shut, you can string the curtains and the sheers on a single rod with the sheers in the middle...same look, lower profile. You can do the same with a valance that covers the middle of the rod with panels on either side. Another alternative would be to look into ceiling mount...perhaps the projection would be less of an issue if it's flush to the ceiling.

Oh I hadn't thought of that. That is definitely another good option! So why do you suggest an outside mount instead of inside mount? I guess one thing to mention is that the top of the window sill sits about 2-2.5' from the ceiling, and I would be hanging the curtains high. So when they get tied back during the day, you would see a bit of the wall above the window. Which I'm fine with. I suppose an outside mount could be more visually appealing because I could hang it high enough to hide the wall and give the illusion of taller windows. But, I just wonder if the shade contraption (I don't know much about shades) would protrude from the wall and interfere with the curtain (again, I suppose I could just ensure my curtain rod leaves enough clearance from the wall).

It’s for a rental. Perhaps you are overthinking it? Generally you need to focus on tenant-proofing. Is this Airbnb or extremely upscale? Furnished?